Retrieval of Total Precipitable Water Using Radiometric Measurements Near 92 and 183 GHz

J. R. Wang NASA/Goddard Space Flight Center, Greenbelt, Maryland

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T. T. Wilheit NASA/Goddard Space Flight Center, Greenbelt, Maryland

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L. A. Chang Science Applications Research, Lanham, Maryland

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Abstract

The strong water vapor absorption line at 183 GHz is explored in this paper for retrieval of total precipitable water in the atmosphere. This strong line has generally been utilized in the past for the profiling of the atmospheric water vapor. It is shown from radiative transfer calculations that, under very dry atmospheric conditions, the radiometric response near this frequency behaves much like that near the 22 GHz absorption line but, with the advantages of an increase in sensitivity and potentially an improvement in spatial resolution. Total precipitable water can be retrieval almost independent of atmospheric temperature profiles under these conditions. The technique is demonstrated with the airborne Advanced Microwave Moisture Sounder (AMMS) which has four channels, three of them centered around 183 GHz (183 ± 2 GHz, 183 ± 5 GHz, and 183 ± 9 GHz) and another at 92 GHz. The calculated sensitivities of radiometric response to total precipitable water are approximately 410, 230, and 130 K (cm)2 g−1 for total precipitable water less than 0.2, 0.3, and 0.5 g (cm)−2 at 183 ± 2 GHz, 183 ± 5 GHz, and 183 ± 9 GHz respectively. The inclusion of the 92 GHz channel extends the range of the retrieval in excess of 1 g (cm)−2 total precipitable water. However, the effect of cloud cover proves to be strong at this frequency and the retrieval has to be applied with care. Two AMMS observations of dry atmosphere following the cold air outbreaks are analyzed to demonstrate the technique.

Abstract

The strong water vapor absorption line at 183 GHz is explored in this paper for retrieval of total precipitable water in the atmosphere. This strong line has generally been utilized in the past for the profiling of the atmospheric water vapor. It is shown from radiative transfer calculations that, under very dry atmospheric conditions, the radiometric response near this frequency behaves much like that near the 22 GHz absorption line but, with the advantages of an increase in sensitivity and potentially an improvement in spatial resolution. Total precipitable water can be retrieval almost independent of atmospheric temperature profiles under these conditions. The technique is demonstrated with the airborne Advanced Microwave Moisture Sounder (AMMS) which has four channels, three of them centered around 183 GHz (183 ± 2 GHz, 183 ± 5 GHz, and 183 ± 9 GHz) and another at 92 GHz. The calculated sensitivities of radiometric response to total precipitable water are approximately 410, 230, and 130 K (cm)2 g−1 for total precipitable water less than 0.2, 0.3, and 0.5 g (cm)−2 at 183 ± 2 GHz, 183 ± 5 GHz, and 183 ± 9 GHz respectively. The inclusion of the 92 GHz channel extends the range of the retrieval in excess of 1 g (cm)−2 total precipitable water. However, the effect of cloud cover proves to be strong at this frequency and the retrieval has to be applied with care. Two AMMS observations of dry atmosphere following the cold air outbreaks are analyzed to demonstrate the technique.

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